Performance of an engine can be enhanced via a turbocharger or a supercharger. The turbocharger or supercharger pressurizes ambient air to increase the density of air entering engine cylinders. The cylinder trapped air amount is increased as the cylinder charge may be denser than that of a non-turbocharged engine. This may allow increased amount of fuel injected to be into the engine cylinder compared to a non-turbocharged engine, hence result in increased torque.
However, during certain conditions (e.g., at low engine speeds and full throttle); boosted engines may be severely knock-limited resulting in reduced torque output. One approach to improve low speed knock-limited torque includes providing variable intake and/or exhaust valve timing. In particular, intake and exhaust valves of a turbocharged engine may be adjusted such that engine output power may be increased when intake and exhaust valves of a cylinder are simultaneously open and when engine intake manifold pressure is higher than engine exhaust manifold pressure. Pressurized air in the engine intake manifold can drive exhaust gases from the cylinder to the engine exhaust manifold so that cylinder fresh charge (e.g. air and fuel) may be increased. Further, by replacing the trapped exhaust gas with fresh air, charge temperatures may be reduced. Consequently, tendency for knock may be reduced.
However, the inventors herein have identified issues with such an approach. As an example, during the overlap period, due to short flow path between the intake and the exhaust valves, the air delivered by the compressor may leak into the exhaust ports before the exhaust gases are completely purged from the cylinder. Consequently, an increased amount of boosted air may be required to purge the chamber of exhaust gases, which may limit an amount of boost that can be provided by the compressor.
Further, in order to maintain a stoichiometric exhaust air-to-fuel ratio, additional fuel may be injected into the cylinder to compensate for the additional air in the exhaust. As a result, exhaust gases may contain high carbon monoxide and hydrogen concentrations, which may combine exothermically with excess oxygen in the additional air, which when oxidized at the catalyst may result in catalyst over temperature conditions.
In one example, the above issues may be at least partly addressed by a method for an engine including one or more four-valve cylinder, comprising: during a first positive valve overlap mode, flowing more blow-through from an intake manifold to an exhaust manifold through a first intake valve and a first exhaust valve than through a second intake valve and a second exhaust valve of the cylinder.
As an example, during engine operation in the first overlap mode, compressed air may be directed from the intake manifold via the first intake valve and hot residual gas may be purged via the first exhaust valve. The first intake valve and the first exhaust valve may be positioned diagonally in a cylinder head. Consequently, during the blow-through, gas may flow through a longer diagonal path.
In this way, by directing the gases to flow through a longer path, exhaust gases may be purged more effectively from the cylinder, and more fresh air may be trapped in the cylinder. The resulting increased cylinder air charge may provide more torque. The increased torque output at low engine speeds may increase time in top gear resulting in improved fuel economy. Further, due to more air being trapped in the cylinder, an amount of air leaking to the exhaust may be reduced. Consequently, cylinder may be operated less rich. The decreased amount of air in the exhaust and the less rich combustion may reduce the amount of fuel and air combining exothermically in the exhaust manifold. As a result, excess increase in exhaust gas temperatures and therefore, excess increase in exhaust catalyst temperatures may be reduced.
Further, as a result of the exhaust gases being effectively purged, an in-cylinder temperature may be reduced. Consequently, the lower in-cylinder temperature may reduce the tendency for knock, which may allow the engine to be operated with more spark advance, which may also contribute to reducing the exhaust temperature.
Taken together, by providing diagonal blow-through, more torque may be available at low speed, fuel economy may be improved, excess heating of exhaust catalyst may be reduced, and reduced incidence of knock may be achieved.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.